Solar energy for NZEB (net-zero energy building)

Net-zero energy building (NZEB) is a term to describe buildings which meet their energy demands on a yearly basis by means of on-site energy production without relying on external power sources like utility grid. In the case of a Norwegian residential building, the paper's authors analyze various configurations of the solar power system installation that meet or come close to NZEB goal.

A typical residential building will need not only electricity but also hot water supply throughout the year. Domestic hot water (DHW) demand can be met by solar thermal (ST) collectors. Another alternative is the photovoltaic thermal (PV/T) module which still has a low market penetration. PV/T modules combine PV panels with ST modules to provide both electricity and hot water in one module. Most PV panels only convert 10 - 20% of solar irradiation into electricity. The rest is dissipated throughout the panels as heat which is detrimental for them in the long run. The promise of PV/T modules is to utilize the excess energy for water heating thus increasing the panel's lifespan. Harnessed waste energy also can improve the panel's efficiency because higher temperatures decrease electricity production. Two main types of PV/T modules are covered and uncovered. Uncovered PV/Ts are basically regular PV panels with fans which reduce the panel's temperature. Uncovered PV/Ts provide equal or more electricity compared to plain PVs and also generate some low-temperature heat. Covered PV/Ts are solar thermal collectors covered with PV cells. The extra layer prevents heat loss from convection so covered PV/T modules provide higher temperature heat than uncovered ones. A recent market study from Germany found 41 producers of PV/T modules, out of which 30 were marketing uncovered modules [1]. Here are the configurations that paper authors devised to explore the possibilities of achieving NZEB design. In cases of 1 - 4 average quality PV and PV/T modules available in the market were used. Case 5 and 6 employed best available PV panels and ST collectors

1. ST and PV (standard config)
2. Only PV/T uncovered
3. PV/T covered and PV
4. Only PV
5. State-of-the-art ST and PV
6. State-of-the-art only PV

It should be noted that all building characteristics remain the same for these 6 cases. The highest result is achieved in case 6. which provides for 87% of the total household electricity demand. The second highest is Case 5. with 82%. From 2 configurations with PV/T Case 2. yielded slightly higher Eexported / Edelivered ratio of 71% vs 68% in Case 3. But covered PV/T provided more output of kW/h by 27%.

References

1. Adam, M., H.P. Wirth, and R. Radosavljevic, PVT-Marketsubersikt. 2014.
2. C. Good; I. Andresen; A.G. Hestnes 2015, Solar Energy For Zero Energy Buildings – A Comparison Between Solar Thermal, PV and Photovoltaic-Thermal (PV/T) Systems